1. Field
This invention relates to a soldering alloy made of tin and an active wetting promoting element such as aluminum and to a soldering technique that is useful for joining a wide variety of hard-to-wet and hard-to-join materials including titanium alloys (e.g., Nitinol), stainless steels, aluminum alloys, carbon steels, glasses, and ceramics.
2. Background
Nitinol is a trade name for a titanium alloy with a composition of nickel 50 atomic % titanium. Also known as xe2x80x9cTee-neexe2x80x9d, xe2x80x9cMemoritexe2x80x9d, xe2x80x9cTinelxe2x80x9d or xe2x80x9cFlexonxe2x80x9d, this alloy is utilized for its superelasticity, or shape memory effect. Shape memory is the ability to fully recover plastic deformation, up to 8%, upon heating above a specific temperature. Superelasticity is the ability to completely recover large xe2x80x9cpseudo-elasticxe2x80x9d strains on the order of 5 to 8% upon removal of the loading stress. This superelasticity and/or shape memory, coupled with Nitinol""s biocompatibility, corrosion resistance, and fatigue resistance, make the alloy a very attractive material for a variety of applications. Although Nitnol can be joined using brazing or welding techniques, it begins to dramatically lose its superelasticity, and/or shape memory characteristics, when heated above approximately 500xc2x0 C., as occurs during welding and brazing. Subsequent heat treatment can only recover a small percentage of the properties.
Although some efforts have been made to use soldering techniques, which by definition use temperatures below about 450xc2x0 C., such efforts have been less than successful. Titanium and titanium alloys are difficult to solder because they form a particularly tenacious surface oxide that is hard to wet. While this oxide imparts these alloys with exceptional corrosion resistance, it also makes them extremely difficult to solder. Although two methods for soldering Nitinol have been used previously: 1) soldering using halogen-based fluxes, and 2) electro/electroless plating, both have significant draw backs.
A number of manufacturers have developed fluxes for use on aluminum alloys, which develop a tenacious surface oxide similar to that of titanium and its alloys. Although these fluxes have been found to be useful with Nitinol, they are typically based on very aggressive halogen bearing inorganic acids that are hazardous to handle and dispose of. During the soldering operation the flux generates large amounts of toxic fume that must be vented to prevent exposure to personnel. The flux residue remaining on the solder joint must also be cleaned off with hot water and mechanical scrubbing. The cleaning water and residue is an environmental hazard and must be handled and disposed of accordingly.
In addition, the flux residues must be cleaned off completely to prevent subsequent corrosion of the solder joint, and potential in-service contamination. Leaching of toxic flux materials from the joint presents a formidable problem in the manufacture of medical devices. The complex joint geometry often necessary in surgical devices makes complete removal of flux residue difficult, time consuming, and expensive. If not completely removed, persistent flux residue can compromise the solder joint integrity and potentially contaminate a surgical patient.
Plating is another technique used for soldering difficult-to-solder alloys. In the case of nickel-titanium, nickel plating can be used. Nitinol can be nickel plated with both electroless and electrolytic processes. Often the part is plated with a secondary layer of a more noble metal, such as gold over the nickel. Soldering can then be done on the plated surface using an appropriate flux. The major drawback to this approach is that plating titanium is an involved and difficult process. Nickel plating is typically a multi-step process involving cleaning, etching, and plating. Plating titanium alloys is even more complex due to their tenacious surface oxides. Often, several intermediate plating steps may be necessary to facilitate the final nickel plating. For most manufacturers, it is very costly to develop extensive in-house plating capabilities and the expertise just to facilitate a soldering process. Furthermore, plating quality can vary greatly and plating vendors are often reluctant to work with titanium alloys due to issues of handling and storing the aggressive chemicals involved such as hydrofluoric acid. An additional drawback is that even though fluxes that are less aggressive than the halogen based ones can be used to solder the plated surface, the flux residues must still be removed completely.
While the concept of ultrasonic soldering has been around for half a century, it has had very limited commercial success. It did show some potential for soldering aluminum heat exchangers, but this effort was largely dropped in favor of several competing approaches. While its use remains relatively limited, the most common commercial application appears to be in pre-tinning, or solder coating, of copper electrical leads and components. Limited success has been achieved in using ultrasonic soldering to solder difficult to solder materials. Although, it has been used in conjunction with indium-based solder alloys to aid slightly in the soldering of oxide ceramics, these joints can also be made easily without the use of ultrasonic soldering. Furthermore, joints made with indium-based alloys, with or without ultrasonic soldering, typically have very low strength, on the order of only several hundred pounds per square inch. Prior attempts to ultrasonically solder titanium and its alloys, using conventional as well as custom solder alloys have shown very limited success producing weak joints that result from a primarily mechanical bond instead of a true chemical bond.
In order to overcome the various problems encountered with prior art methods of joining hard-to-wet materials, it is an object of the present invention to provide a soldering method and solder alloy for wetting and joining these hard-to-join materials.
It is an object of the present invention to join hard-to-join materials at a low temperature.
It is an object of the present invention to join hard-to-join materials without the need for corrosive fluxes.
It is an object of the present invention to avoid the production of hazardous workplace fumes and joining by-products
It is an object of the present invention to avoid the production of environmentally dangerous fumes and joining by-products.
It is an object of the present invention to join hard-to-join materials without the need for tedious and complex plating steps.
It is an object of the present invention to join hard-to-join materials without the need for use of corrosive plating chemicals.
It is an object of the present invention to join hard-to-join materials using a minimum of processing steps.
It is an object of the present invention to join nitinol parts without loss of superelasticity or shape memory characteristics.
It is an object of the present invention to clean hard-to-join materials without the use of a flux.
It is an object of the present invention to provide high-strength joints for hard-to-join materials.
It is an object of the present invention to join hard-to-join materials using inexpensive equipment.
It is an object of the present invention to join hard-to-join materials with little if any joint cleanup after the joining process.
It is an object of the present invention to join hard-to-join materials with no flux residue cleaning after the joining process.
It is an object of the present invention to join hard-to-join materials without the use of a vacuum, or reducing gas environment.
It is an object of the present invention to form solder wetted areas on hard-to-wet materials.
It is an object of the present invention to provide a tin-based alloy solder with an active wetting promoting chemical element.
It is an object of the present invention to use aluminum as an active wetting promoting element in a tin alloy.
The foregoing and other objects, features and advantages of the invention will become apparent from the following disclosure in which one or more preferred embodiments of the invention are described in detail. It is contemplated that variations in procedures may appear to a person skilled in the art without departing from the scope of or sacrificing any of the advantages of the invention.
To meet these objects a novel and useful solder alloy has been developed. The soldering alloy consists essentially of a tin base and an active wetting promoting element such as aluminum in the presence of ultrasonic energy. Typically aluminum may be used in amounts ranging from 0.1 to 20 wt % with 2-3 wt % being readily prepared by dissolving aluminum in molten tin until saturation occurs. The tin-aluminum alloy has been found to readily wet a wide variety of materials with tenacious surface oxides that previously have been wettable only through the use of highly corrosive flux materials. The feature of using a fluxless soldering alloy has the advantage of eliminating hazardous soldering plumes formed during the soldering process and also eliminating the corrosive flux residue left after the soldering operation is complete. Other elements such as silicon, magnesium, calcium, titanium, hafnium, zirconium, and zinc may be used in place of or in addition to aluminum.
This solder alloy has been found to easily wet a wide variety of previously hard to wet materials including, but not limited to, nickel-titanium alloys such as nitinol, aluminum and aluminum alloys, stainless steels, carbon steels, glasses, ceramics including oxide and carbide ceramics, copper and copper alloys, nickel and nickel alloys including nickel-iron alloys. Once wetted, a wide variety of joints are easily obtainable including nitinol to nitinol, glass to glass, ceramic to ceramic, glass to metal, metal to metal, alloy to alloy, glass to alloy, ceramic to glass, among others.
The wetting capabilities of the soldering alloy is particularly effective when used In the presence of ultrasound to wet hard-to-wet work pieces such as those with a tenacious surface oxide or other hard to remove surface layer. The solder alloy of the present invention is heated to a molten state and contracted with an area of the hard-to-wet work piece and ultrasound is applied to produce a solder wetted joint area on the work piece. Such a wetted material is useful in itself as a protective coating or as readily adhered base surface capable of receiving other coatings. The wetting process may be carried out in an inert atmosphere to prevent undue oxidation of the molten solder and the work piece as it is removed from the soldering environment. The solder alloy of the present invention may be applied with soldering irons and soldering pots, especially those equipped for use with ultrasound.
In typical joining applications of hard-to-wet materials such as those with a tenacious surface oxide, an area of each part to be joined is placed in close proximity or in contact with each other and the areas to be joined are heated to produce a molten tin-alloy solder. The molten tin-alloy solder is contacted with at least one of the parts to be joined in the area of joining and ultrasonic energy is applied to the molten tin-aluminum solder in contact with the part to be joined until the area of each part to be joined is wetted with the molten tin-alloy solder. Once the parts to be joined are wetted with the solder in the area of the joint, i.e., the bond region, the parts to be joined are allowed to cool to form a soldered joint in the bond region between the parts to be joined.
Alternatively, the joining method may be practiced by heating one of the parts to be joined in the area of joining to the meting point of the tin-alloy solder. Molten tin-alloy solder on a heated soldering iron is then contacted with the part in said area of joining and ultrasonic energy applied until the area of said part to be joined is wetted with said molten tin-alloy solder. The process is repeated in a similar fashion for a second part. After both parts are wetted with the tin-alloy solder, the two parts are placed together and reheated to allow the tin-alloy solder from each part to flow together after which the parts are cooled to form a joint between the first and second parts.
In a third embodiment, areas of the parts to be joined are secured in proximity with each other, that is, dose to or in contact with each other, and the areas immersed in molten tin-alloy solder to which ultrasonic energy is applied until the area of each part to be joined is wetted with molten tin-active element alloy solder. The parts are then withdrawn from said molten solder and allowed to cool to form a solid joint in the bond region between the parts.
In yet a fourth embodiment, areas of the parts to be joined (in the area of the joint) are immersed in molten tin-active wetting promoting chemical element alloy solder and ultrasonic energy applied to the molten tin-alloy solder until the joint areas of each part are wetted with the molten tin-alloy solder. The parts are withdrawn from the molten solder and the wetted areas of the parts placed in contact with each other and the tin-alloy solder flowed together with additional heat after which the parts are allowed to cool to form the desired joint.
It is to be realized the both work pieces to be joined do not have to be hard-to-wet materials. The solder will also wet conventional materials and allow their joining to hard to wet materials. When the work pieces are first wetted and allowed to cool followed by bond region positioning of the respective areas of the work pieces, additional solder alloy may be applied to the bond region as it is heated to produce the desired joint. Additional ultrasonic energy may also be used during the reheating process. It is also possible to apply and use conventional solders during the reheat process.
The foregoing and other objects, features and advantages of the invention will become apparent from the following disclosure in which one or more preferred embodiments of the invention are described in detail and illustrated in the accompanying drawings. It is contemplated that equivalent variations in procedures, alloy and soldered material compositions and arrangement of parts may appear to a person skilled in the art without departing from the scope of or sacrificing any of the advantages of the invention.